This invention concerns a device and method for measuring and continuously monitoring the concentration of a colored component contained in a flowing liquid or gas stream.
Potassium permanganate is often used in water purification systems in quite small concentrations, and it is desirable in such systems to continuously monitor the permanganate concentration.
In refinery products such as gasoline, color concentration is often a specification to be met by the final product, and the quality of final product can often be monitored by measuring the color of intermediates or final product.
This invention is especially suited to the above applications and to other applications in which it is desired to continuously measure the concentration of one or more colored components in a flowing stream containing such components.
Many types of colorimetric devices have been devised in the past to measure the light transmission characteristics of solid, liquid and gaseous materials over wide ranges of light transmission value and wave length. The light absorption of a colored component in a stream varies with the concentration of that component in the stream according to known physical laws. One method for measuring concentration of a colored substance is to measure the light absorbance of the solution by passing light through a predetermined thickness of the solution. Such method is generally unsuitable for streams containing impurities which can affect both the absorbance characteristics of the sample and the intensity of the light passing through the sample.
Dual beam colorimetric devices are also known, in which one beam is passed through the sample to be measured and the second beam is passed through a reference sample of known concentration. Comparison of the output beams from both samples may be detected by photoconductor devices used as resistance elements in a null circuit, and the difference in output of the photoconductors due to a difference in concentration of colored component in the stream to be measured and the reference sample may be detected and monitored by measuring the degree of imbalance of the null circuit. In these dual beam devices, impurities in the stream to be measured other than the component to be measured can cause loss of sensitivity and erroneous readings.
All known prior art colorimetric devices employ optical quality lenses to direct the light beam(s) through the sample to be monitored. Here, also, impurities cause erroneous readings in either single or dual beam devices since an impurity particle will cast a shadow upon the receiving photoconductor causing a decrease in intensity of light reaching that photoconductor which is totally unrelated to the absorbance of light by the colored component of interest.
Specific colorimetric devices which were found during a search of prior art related to this invention include those disclosed in the following patents.
U.S. Pat. No. 3,062,963 (Douty) discloses a method of monitoring colored fluids in which light absorbance of a sample is measured at a wave length of light which is materially absorbed by both the key colored substance to be measured and by the impurities in the stream, and also at a wave length of light which is absorbed by only the impurities. A comparison of the two absorbances provides a measure of the concentration in the stream of a key colored substance.
U.S. Pat. No. 2,895,055 (Crane et al.) discloses a method and apparatus for the colorimetric determination of the concentration of iron in process streams such as those encountered in the production of sodium hydroxide. The method is performed by withdrawing a sample from the process stream, adding color developing agents to the sample, transporting the sample plus developing agents to a transparent cell, and passing two light beams through the cell. One of the beams is of a color which is not absorbed by the sample and the other beam is of a color which is strongly absorbed by the sample. After passage through the sample, the beams are focused on separate photoelectric devices which are connected to a null balance circuit which measures the difference in output of the photoelectric cells as a function of the concentration of the color producing material (iron).
U.S. Pat. No. 2,737,591 (Wright et al.) discloses a method and apparatus for the analysis and control of organic systems in which a single beam of infrared light is passed through a sample cell containing a mixture to be analyzed. The beam is split by a beam splitter after passage through the sample, one split beam being directed to a light-responsive detector which is strongly absorptive of light of a distinctive wave length which is absorbed more strongly by the component to be determined in the analysis than by the other components of the mixture, and directing the second split beam to a light responsive deflector which does not strongly absorb light of the said distinctive wave length, but does absorb light of other wave lengths. Each detector develops an electric current or potential to an extent proportionate to the absorbed light energy, which can be monitored to indicate the concentration of the component to be determined.
U.S. Pat. No. 3,435,209 (Keahl) discloses a radiant energy analyzer by which radiation is directed alternately along first and second beam paths in each of which are located separate, independently operated wave length selectors such that radiant energy of differing wave lengths may be selected in each beam path. The beams are then recombined and focused onto a detector connected to an optical null servo loop. A sample cell can be placed in the beam path and certain optical characteristics of the sample can be measured.
U.S. Pat. No. 3,844,661 concerns an optical cell containing a fluid sample, the cell having a wiper blade mechanism to keep the cell clean, and selected wave lengths passed through the cell are measured by photosensor means.
U.S. Pat. No. 3,381,135 (Keller) discloses a double beam spectrophotometer utilizing an ultraviolet source of radiation of a relatively narrow band width of wave lengths to test the absorbance of a solution. The beam of light from the source is split into two beams, a reference sample of known concentration of material in a liquid is placed in the path of the first beam of light, and the second beam is caused to pass through a wedge of known concentration of material in a liquid which can be moved to cause the path length of light through the wedge to vary. The second beam of light also passes through a liquid of unknown concentration of material which is under test. The output from the spectrophotometer is used to control the absorbance of the liquid under test to maintain a predetermined necessary concentration of a material in the liquid under test.
U.S. Pat. No. 3,076,375 (Donnell) discloses a colorimeter having a linear absorbance scale. In this colorimeter, a beam of light is projected through a sample to be analyzed for color. The light transmitted through the sample is passed through a color filter which transmits substantially only the desired wave length, and then to a light detecting element or photocell. This cell is connected to an alternating current bridge circuit from which the output is fed to a logarithmic amplifier. The output of the amplifier is fed to a recorder having a linear scale, the reading of the scale being linearly related to the color concentration of the sample.
U.S. Pat. No. 3,652,861 (Engholdt) discloses a control device for detecting the hardness level of water. The device includes sampling means for extracting a sample of water from a water system and mixing the sample with a reagent which is formulated to cause the sample to change color at a predetermined hardness level. The mixed sample is exposed to a detection device which includes a bridge circuit and a voltage sensitive means connected to the bridge circuit for sensing an unbalanced condition in the bridge circuit. A pair of photoconductive cells are connected in series in one leg of the bridge circuit. One of the cells is positioned to receive light which has passed through the sample and the other cell is positioned to receive light which has passed through a filter element corresponding in color to a sample which is below a predetermined hardness level. When the hardness of the sample is below a preselected level, it will take on the color of the filter element and the light falling on both cells will be of substantially the same color and intensity. Thus, the bridge circuit will remain in a balanced condition. When the sample is above the preselected hardness level, it will assume a color different from that of the filter element. When this happens, the color and intensity of the light falling on the two cells will be different causing the bridge circuit to become unbalanced. The voltage sensitive means will sense this unbalanced condition and produce a signal which may be used to initiate the regeneration cycle of a water softener, for example.
U.S. Pat. No. 3,723,062 (Dahms) discloses a method and apparatus for colorimetric titration using an indicator which changes from one color to another at or near a titration endpoint, and an optical endpoint detection system which generates signals responsive to the relative concentration of the different colored forms of the indicator.
U.S. Pat. No. 3,308,712 (Kay) discloses a transducer for spectrum analysis including a polychromatic light source, a dynamic filter, and a detector. The dynamic filter allows light of one frequency to pass, the partcular frequency allowed to pass being variable as a function of time. When used as an absorption spectrometer, the light which passes through the dynamic filter is directed through a sample and then to a detector. The detector produces a signal which represents the absorption spectrum of the sample. When used to detect the emission spectra of the sample, the light source is replaced by the luminous sample and the light from the sample is directed through the dynamic filter to the detector.
None of the aforementioned references disclose the device and method of the invention herein which are described in detail and claimed hereinbelow.